Thermally stable coil assembly for magnetic susceptibility logging

ABSTRACT

A borehole magnetic susceptibility apparatus is disclosed which includes a transmitter search coil, a receiver search coil, both wound over a low thermal co-efficient core element such as glass, an AC power supply coupled with the transmitter search coil for generating an alternating magnetic field which after passing at least in part through the earth formation couples magnetically with the receiver search coil for inducing therein a voltage signal dependent in part on the magnetic susceptibility of the formation and which is in quadrature phase relationship with the transmitter coil current. The apparatus includes a nulling coil having a primary winding connected in series with the transmitter coil and a secondary winding connected in series opposition with the receiver coil to render detectable the small variations of the quadrature phase signal representing the magnetic susceptibility. A thermally stable and shock resistent subassembly of the transmitter and receiver search coils is provided for improved environmental stability of the tool. The method of subjecting the earth formation to an alternating magnetic field to induce therein a field which depends on the magnetic susceptibility of the formation, detecting the induced magnetic field by permitting it to act on a receiver element to induce therein a first signal which depends on the magnetic susceptibility of the formation, generating a reference second signal representing the susceptibility of a reference medium, and generating a third signal corresponding the difference between the first and second signal whereby the third signal represents the magnetic susceptibility of the formation with respect to the magnetic susceptibility of the reference medium.

[451 Apr. 18,1972

coil, both wound over a low thermal caefi'icient core element such asglass, an AC power supply coupled with the transmitter search coil forgenerating an alternating magnetic field which after passing at least inpart through the earth formation couples magnetically with the receiversearch coil for inducing therein a voltage signal dependent in part onthe magnetic susceptibility of the formation and which is in quadraturephase relationship with the transmitter coil current. The paratusincludes a nulling coil having a primary winding connected in serieswith the transmitter coil and a secondary winding connected in seriesopposition with the receiver coil to render detectable the smallvariations of the quadrature United States Patent Dowling et al.

[54] THERMALLYSTABLECOIL [73] Assignee:

[22] Filed:

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ln prospecting for oil, gas deposits, and other minerals, it isessential to have as much information as possible about the earth'sstrata at various depths to help identify the materials present and theage of the various formations. One commonly used method is boreholeinduction logging for determining the electricalconductivity of theformations such as is disclosed by G. Attali in U.S. Pat. No. 3,327,203.Another method is logging to determine the magnetic properties of theearths material such as the magnetic susceptibility or permeability.

While the infonnation obtainable by the latter method is highlydesirable, serious problems are encountered when prior art devices areused for magnetic logging. The reason is that the magnetic anomaly ofearth formations is generally of a smaller magnitude than is theelectrical conductivity anomaly. Therefore, logging signals representingthe magnetic proper ties are generally of a much smaller order ofmagnitude. As a Furthermore, such prior art devices have been directedtoward the problem of obtaining electrical conductivity data.Consequently, they are constructed to treat the signals representing themagnetic properties of the formations as background noise.

In magnetic susceptibility logging as is proposed herein a solenoidallywound transmitter coil having its axis maintained approximately parallelwith the borehole is excited by an AC voltage so that an alternatingmagnetic field is applied to the earth s strata. A receiver coil issuitably positioned in the tool where it is subjected to the magneticfield as it is altered by the magnetic anomalies of the materialsurrounding the borehole. The magnetic field results in electricalcurrents which flow in a more or less circumferential pattern around theborehole and which vary with the conductivity of the formation. As aresult of these currents there is induced in the receiver coil a voltagesignal which is in phase with the transmitter coil current. Inquadrature relationship with this voltage signal is the magneticallyinduced voltage signal of the receiver coil due to its magnetic couplingwith the transmitter coil. A significant portion of that coupling isthrough the earth's formation surrounding the borehole. The magneticanomalies, hence, appear as variations of the quadrature signal. Thesevariations are much smaller than the in-phase signal representing theconductivity of the formation. The orders of magnitude which can beexpected are approximately as follows. A transmitter coil current ofapproximately 300 ma. can result in an inphase conductivity signal inthe order of magnitude of about 500 mv. and an induced quadrature phasesignal of about 28 mv. with variations of approximately 0.05 mv. due tothe magnetic anomalies of the formation. It can be seen that in order torender detectable the small signal variations representing the magneticanomalies it is desirabie to subtract from the total magneticallyinduced signal a constant AC signal approximately equal to what themagnetically induced signal would be if the logging tool is surroundedby a medium of known magnetic properties, such as, for example, air. Toprovide such a subtraction signal we propose to utilize a nulling coilin a novel manner resulting in an accurate subtraction signal which hasno detrimental effect on the quadrature signal representing the magneticanomalies.

The use of a conventional transformer for this purpose such as isdisclosed in U.S. Pat. No. 3,327,203 is unsatisfactory for magneticproperty logging since such :a transformer would be sensitive totemperature variations and! to the earths magnetic field to an extentwhich would obscure the variations of the magnetically induced signalsought to be detected. We expect that such variations can result inunsubtracted residual signals in the order of magnitude of approximately1 mv. Even greater residual signals can be expected if other prior artnulling devices are used. Comparing these residual signals with theaforementioned magnetic anomalies signal it can be seen that prior artnulling devices and techniques are inadequate for magnetic propertylogging.

. We have found that a nulling coil constructed and utilized ashereinafter disclosed solves the aforementioned problems and effectivelyeliminates any interference from the transmitter coil field. Also, thenulling coil we propose reduces sensitivity to variations of the earthsmagnetic field to a tolerable level. It is interesting to note that coilpairs are usually wound over highly permeable core elements. The use wepropose however involves construction of the coil such that it tends toprovide a high degree of temperature stability and interferencerejection.

Further improvements we propose include a unique design of thetransmitter and receiver coils to provide high environmental stabilitywith a desired magnetic penetration in the formation.

Our invention provides a logging apparatus and method by which amagnetic property signal is generated of sufiicient strength andenvironmental stability such that any signals representing theelectrical conductivity of the formation can be excluded by presentlyknown phase detection equipment, whereby the magnetic susceptibility canbe reliably logged with results heretofore unobtainable.

SUMMARY In accordance with a preferred aspect of the invention there isprovided a magnetic susceptibility logging apparatus which includes atransmitter search coil and a receiver search coil both solenoidallywound over a core material having a relatively low coefficient ofthermal expansion, and an electrical power supply for providingalternating current power to the transmitter search coil for generatingan alternating magnetic field. The apparatus also includes a loggingtool case for supporting the transmitter search coil in the borehole ina position such that the earth formation is subjected, at least in part,to the alternating magnetic field. The receiver coil is magneticallycoupled with the transmitter coil such that the magnetic field afterpassing through the earth formation acts upon the receiver coil andinduces therein an alternating voltage signal which is dependent in parton the magnetic susceptibility of the formation and which is inquadrature phase relationship with the alternating current in thetransmitter coil. The apparatus also includes a nulling coil having aprimary winding connected in series with the transmitter coil and asecondary winding connected in series opposition with the receiver coil.The nulling coil is mounted in the casein a position such that theaforementioned magnetic field has substantially no effect on the primaryand secondary windings. The voltage induced in the secondary issubtracted from the receiver coil voltage to render detectable theportion of the receiver coil voltage representative of the magneticsusceptibility of the earth formation.

In accordance with another aspect of the invention an improvedconstruction of the search coils is provided wherein they aresolenoidally wound over a common mandrel of a material having arelatively low coefficient of thermal expansion such as, for example,glass. A preferred axial spacing on the mandrel is provided in order toobtain a desired penetration into the formation. Also, supporting endcaps of boron nitride material are provided for the windings of thetransmitter and receiver search coils which in conjunction with theglass mandrel result in excellent temperature stability of the loggingtool.

Another aspect of the invention resides in a method for providing asignal representative of the magnetic susceptibility of the earthformation with respect to the susceptibility of a reference mediumincluding the steps of subjecting the earth formation to an alternatingmagnetic field for inducing therein an alternating field which dependsupon the magnetic susceptibility of the formation, detecting the inducedmagnetic field by permitting the field to act on a receiver elementplaced in the vicinity of the formation for inducing in the receiverelement a first alternating voltage signal which depends on the magneticsusceptibility of the formation and generating a reference secondalternating voltage signal corresponding to the value the induced firstsignal would assume if the first signal were provided with respect to areference medium of known magnetic susceptibility. The method includesthe further step of providing a magnetic susceptibility output thirdsignal corresponding to the difference between the first and secondsignals whereby the output third signal is representative of thesusceptibility of the earth formation with respect to the susceptibilityof the reference medium.

In view of the foregoing it is an object of the invention to provide amagnetic susceptibility logging tool.

Another object of the invention is to provide an improved logging toolof sufiicient sensitivity to provide data of the magnetic susceptibilityof the earth formation adjacent a borehole.

Another object of the invention is to provide a logging tool of improvedenvironmental stability which is suitable for the purpose of magneticsusceptibility logging.

Another object of the invention is to provide a method for generating asignal representative of the magnetic susceptibility of the earthformation adjacent a borehole with respect to the susceptibility of aknown medium.

These and other object, advantages and features of the invention, willbe more fully understood by referring to the following descriptions andclaims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified cross-sectionalview of a logging tool illustrating features of the invention.

FIG. 2 is a vector diagram illustrating the phase relationships betweenthe more significant voltages and currents of the transmitter andreceiver coils.

FIG. 3 is a schematic diagram partly in block form illustrating electriccircuit features of a logging system in accordance herewith.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 a longcylindrical tool case is provided which is made of a suitablenon-metallic material having reasonably high mechanical strength such aspaper impregnated bakelite. A nulling coil 11 is provided mounted in theupper portion of the case. A solenoidally wound transmitter coil 12 isprovided in the lower portion of the logging tool. A solenoidally woundreceiver coil 13 is located in the central portion of the case 10. Toprovide the'required subtraction voltage the nulling coil 11 includes aprimary winding wired in series with the transmitter coil 12 and asecondary winding wired in series opposition with the receiver searchcoil 13. The core element of the nulling coil is toroidally shaped andmade of boron nitride. The primary winding is toroidally wound over thecore element and the secondary winding is in turn wound over theprimary. The construction and mounting of the nulling coil is criticalsince any relative movement between the primary and secondary windingsresults in a change of the voltage in the secondary which can be largeenough to be misinterpreted as a change of the magnetic properties ofthe earth formation. Thus, it is important that a material having a lowcoefficient of thermal expansion be used for the core element.Accordingly, a non-ferrous material is preferred. We have found thatsuccessfuloperation can be expected if the material has a coefiicient ofthermal expansion less than 1.9 X 10* inches per inch per F. Also, it ispreferable that the core material be machineable since its fabricationinvolves boring and a turning operation. Accordingly, the use of boronnitride is preferred.

The nulling coil 1 1 is mounted with its toroidal axis approximatelyorthogonal to the magnetic axis of the transmitter coil 12. We havefound that in this position any magnetic interference from thetransmitter coil is maintained to a minimum since any inducedinterfering voltages tend to cancel themselves. Also, we have found thata further advantage of a toroidal nulling coil having a boron nitridecore is that it is relatively impervious to the earths magnetic field.Accordingly, the use of shielding is not required.

A housing member 14, is provided, in which is mounted the nulling coil11 by conventional techniques. The external shape of the housing 14 iscircular and is sized to fit snugly within the inside diameter of thecase 10. Also, the housing 14 includes a cut-out region in which ismounted a conventional solid state amplifier and phase detector 15,which amplifies the magnetic susceptibility signal. Extending from thelower portion of the housing 14 is a cylindrical extension havingcircumferential serations. Over this extension is vulcanized a neoprenerubber support member 16. Beneath the rubber support 16 is provided amandrel end cap 17 which on its upper portion also includes acylindrical extension having circumferential serations and which is inturn vulcanized in the lower portion of the rubber support member 16.There is provided in the lower portion of the end cap 17 a bore having apair of internally recessed grooves 19. It is preferred that the end cap17 be made of a non-metallic material having reasonably high mechanicalstrength such as a linen impregnated bakelite. A pyrex-type glassmandrel 20 is provided which closely fits in the bore of the end cap 17and extends downwardly therefrom in approximate axial alignment with thecase 10. The use of pyrex glass is preferred due to its very lowcoefficient of thermal expansion and relatively high mechanicalstrength. Accordingly, any thermally caused relative movement betweenthe transmitter and receiver coils is kept to minimum. We have foundthat successful operation results if a material is used which has athermal coefficient less than about 1.6 X 10 inches per inch per F. Itis nevertheless preferred that a relatively strong glasseous material beused having a thermal coefficient in the range of about 0.5 X 10 to 1.5X 10 inches per inch per F. The mandrel 20 is fixed to the bore of theend cap 17 by utilizing an appropriate cement in the grooves 19.

The glass mandrel 20 extends downward through a major portion of thebalance of the logging tool. The transmitter coil 12 is solenoidallywound over the lower portion of the glass mandrel 20 between a pair ofsupporting end caps, 21 and 22, which are of boron nitride material. Theend caps, 21 and 22 are fitted closely over the mandrel 20 and includecounterbored recesses, 23 and 24 respectively, where a suitable cementis used for fixing the end caps to the glass mandrel. The transmittercoil 12 is tightly wound between the boron nitride end caps whosecoefficient of thermal expansion is sufficiently close to that of theglass mandrel so that thermally caused movement of the transmitter coilwindings due to self-heating and the temperature extremes encountered inthe borehole is nearly eliminated. The receiver search coil 13, issimilarly mounted between boron nitride end caps 25 and 26 and islocated in the central portion of the logging tool axially spaced apartfrom the transmitter search coil a predetermined distance to obtainmagnetic coupling therewith through the earths formation surrounding theborehole.

The lowerend of the mandrel 20 is supported by a neoprene rubber plug 27which fits closely in the case 10. To provide radial support for themandrel 20, several cylindrically shaped supports of neoprene rubber areprovided which fit closely over the mandrel and also in the case 10.These supports are axially spaced at various points along the length ofthe manthe radial and axial directions in the rubber drel. Three ofthese supports, 28, 29, and 30 are shown. The

lower end of the logging tool terminates in a conventional conicallyshaped and weighted tool end 31, which is conventionally fastened to thecase 10.

Generally it is preferred to provide a sufficient number of windings ofthe transmitter coil 12 to generate as strong a magnetic field aspossible within the space available in the logging tool. However, thereare many limitations which constrain the design of the transmitter coil.Thus, if relatively high current and large number of windings are usedfor this coil there is a tendency for excessive self-heating whichadversely affects the accuracy of the tool. We have found that operableresults are obtained if the transmitter coil is designed to provide atotal magnetic field strength passing through the core in the range ofto 60 Oersteds RMS. Best results are obtained when it is designed toprovide a field in the range of about 35 to 40 Oersteds. We considerthis the best compromise which results in a sufficiently strong magneticfield to obtain reliable susceptibility data and which limits theaforementioned selfheating problem. Accordingly, if a 26 AC volts powersupply at the logging tool is provided it is preferred that thetransmitter coil have approximately 1680 turns of No. 22 AWG wire. Thisresults in transmitter coil currents in the range of 250 ma. to 300 ma.

The receiver coil 13 must be axially spaced from the transmitter coil asufficient distance to permit magnetic coupling with a meaningfulpenetration of the field in the formation. Yet the magnetic fieldstrength at the receiver coil must be sufiiciently strong to providedetectable magnetic susceptibility signals. We have found that an axialdistance between the nearest windings of the transmitter and receivercoils of about I 200 to 400 percent of the axial length of thetransmitter coil is preferred. To provide the desired sensitivity of thereceiver coil it is preferred that approximately 9,600 turns of No.

AWG wire be used for this coil. This preferred configuration results inan induced receiver coil voltage in the range of 25 to 28 mv. RMS whenthe tool is exposed to air.

It should be noted that an important aspect of the constructiondescribed above, is that the mandrel 20 with the associated transmitterand receiver coils is a resiliently suspended sub-assembled unit. Thatis, it is suspended both in mounting provisions discussed. Also, thissub-assembly is least sensitive to temperature variations, due to thethermal compatabitity of the boron nitride end caps and the glassmandrel. This construction results in environmental stability of thelogging tool both with respect to temperature variations and vibrationsencountered in the borehole.

It should also be noted the use of both ferrous and non-fen rous metalsshould be avoided anywhere in the aforementioned assembly of mandrel andcoils. The reason is that any metals would tend to confine thetransmitter coil magnetic field and would also result in excessive eddycurrent losses in the metallic parts. Also, such metallic parts wouldpossess magnetic susceptibilities large in comparison with thesusceptibility of the earth formation and their use would accordinglyreduce the sensitivity of the logging tool.

The phase relationships between the various currents and voltages isbest understood by referring to FIG. 2, in which is shown a verticalvector e,, representing the transmitter coil excitation voltage andwhich is drawn vertically, representing 0 phase. This results in atransmitter coil current, i,, which lags e, by a phase angle of 90. Inphase with i, is h, representing the magnetic field generated by thetransmitter coil. Lagging h, by a further 90 phase relationship is irepresenting the eddy currents in the earth formation induced by therate of change of h,. This, in turn, gives rise to a voltage induced inthe receiver coil e which is in quadrature relationship with i Thissignal is directly related to the conductivity of the earth formation.In phase with i is h, representing the small magnetic field associatedwith the eddy currents in the earth formation. In quadraturerelationship with h, is the magnetically induced receiver coil voltage ewhich is induced by the rate of change of h,. The magneticsusceptibility voltage appears as a small component 'part of c and isidentified e Accordingly, the magnetic susceptibility voltage to bedetected is in quadrature relationship with the transmitter coil currenti Referring now to FIG. 3, which is a schematic diagram of the loggingtool and the associated electronics, partly in block diagram form, aconventional oscillator and power amplifier 35 is provided which islocated overhead for transmitting power to the logging tool throughsuitable cable means. The oscillator, and power amplifier 35, areadapted to provide electrical power having a frequency of about 1,000Hz. and a voltage of about 26v. RMS to the logging tool. Wired in serieswith the power amplifier 35 are the transmitter coil 12, and the primarywinding 36 of the nulling coil 11. The secondary winding 37, of thenulling coil, is connected in series opposition with the receiver coil13 and provides a net signal which is carried to a phase detector andamplifier 15 which is mounted in the logging tool. The phase detectorand amplifier rejects the component of the receiver coil voltage whichis in phase with the transmitter coil current and amplifies the netquadrature phase component which represents the magnetic susceptibilityof the formation. This amplified signal is transmitted through suitableelectrical wiring to the surface where it is recorded by a recorder 40.

It is important, in order for the magnetic susceptibility signal to havea meaningful value, that the nulling coil include the proper number ofwindings in the primary and secondary such that the net voltage providedby the receiver coil be as low as possible to represent the magneticsusceptibility of the formation. Thus, it is preferred that the numberof turns of the nulling coil windings be determined such that the netreceiver coil voltage be near zero when the logging tool is in a mediumof known magnetic properties such as in air. By this method a meaningfulcalibration is obtained of bility signal.

When the invention has been described with a certain degree ofparticularity, it can nevertheless, be seen by the examples hereinaboveset forth that many modifications and variations of the invention may bemade without departing from the spirit and scope thereo We claim:

1. Apparatus for logging the magnetic susceptibility of the earthformation adjacent a borehole comprising:

a. a solenoidally wound transmitter search coil including a glass corewhich has a coefficient of thermal expansion no greater than 1.6 X 10inches per inch per F;

b. an electrical power supply for providing alternating current powerfor said transmitter search coil whereby an alternating magnetic fieldis generated;

0. means including a logging tool case for supporting said transmittersearch coil in said borehole in a position such that the earth formationadjacent said borehole is subjected at least in part to said magneticfield;

d. a solenoidally wound receiver search coil including a glass corewhich has a coefficient of thermal expansion no greater than 1.6 X 10inches per inch per F. said receiver search coil being mounted in saidcase (c) in a position axially spaced apart from said transmitter coil(a) a predetermined distance to obtain magnetic coupling with saidtransmitter coil at least in part through the earth formation adjacentsaid borehole whereby said alternating magnetic field induces in saidreceiver coil a first alternating voltage signal dependent in part onthe magnetic susceptibility of said earth formation and which is inquadrature phase relationship with the alternating current in saidtransmitter coil;

e. a nulling coil having a primary winding connected in series with saidtransmitter coil and a secondary winding connected in series oppositionwith said receiver coil said nulling coil being mounted in said case (c)in a position such that said alternating magnetic field hassubstantially no affect on said primary and secondary winding thereofwhereby the induced voltage in said secondary winding the magneticsusceptitends to be subtracted from said receiver coil voltage renderingdetectable the portion of said receiver coil voltage representative ofthe magnetic susceptibility of the earth formation; and,

f. phase sensitive detecting means coupled to said receiver search coilfor deriving a magnetic susceptibility signal voltage corresponding tothe portion of said receiver search coil voltage representative of themagnetic susceptibility of the earth formation in quadrature phaserelationship with the alternating current in said transmitter coil.

2. The apparatus of claim 1 wherein the core of said transmitter searchcoil (a) and said receiver search coil (d) comprises a mandrel corecommon to both of said coils wherein each of said coils is a pluralityof electrical windings on said mandrel and wherein the axial distance ofthe nearest winding of said receiver search coil to the nearest windingof said transmitter search coil is in the range of about 200 to 400percent of the axial length of said transmitter search coil on saidmandrel.

3. The apparatus of claim 1 wherein each of said transmitter andreceiver search coils includes a pair of end caps of boron nitridematerial each having an opening adapted to fit closely over said mandrelfor providing axial support for the windings of said transmitter andreceiver coils, said end caps being cemented to said mandrel.

4. The apparatus of claim 1 wherein said transmitter coil (a) includes asufficient number of windings to provide said magnetic field as a totalmagnetic field the strength of which is about 20 to 60 Oersteds.

5. Apparatus for logging the magnetic susceptibility of the earthformation adjacent a borehole comprising:

a. a glass mandrel having a coefficient of thermal expansion no greaterthan 1.6 X 10 inches per inch per F;

b. a transmitter search coil including a multiplicity of electricalwindings solenoidally wound over said mandrel;

c. means including a logging tool case for supporting said mandrel insaid borehole in a position such that the magnetic axis of saidtransmitter search coil is approximately parallel with the axis of saidborehole;

. an electrical power supply for providing alternating current power forsaid transmitter search coil whereby an alternating magnetic field isgenerated which passes at least in part through said earth formation;

. a receiver search coil solenoidally wound over said manf. at least oneend cap of boron nitride material for each of said search coils forproviding axial support for the electrical windings thereof, each ofsaid end caps including an opening adapted to fit closely over saidmandrel;

. a nulling coil having a primary winding connected in series with saidtransmitter coil and a secondary winding connected in series oppositionwith said receiver coil said nulling coil being mounted in said case (c)in a position such that said alternating magnetic field hassubstantially no affect on said primary and secondary windings thereof,whereby the induced voltage in said secondary winding tends to besubtracted from said receiver coil voltage rendering detectable theportion of said receiver coil voltage representative of the magneticsusceptibility of the earth formation; and,

. phase sensitive detecting means coupled to said receiver search coilfor deriving a magnetic susceptibility signal voltage corresponding tothe portion of said receiver search coil voltage re presentatiye of themagnetic susceptibility of the earth ormatlon 1n quadrature phaserelationship with the alternating current in said transmitter coil.

* l IK

1. Apparatus for logging the magnetic susceptibility of the earthformation adjacent a borehole comprising: a. a solenoidally woundtransmitter search coil including a glass core which has a coefficientof thermal expansion no greater than 1.6 X 10 5 inches per inch per *F;b. an electrical power supply for providing alternating current powerfor said transmitter search coil whereby an alternating magnetic fieldis generated; c. means including a logging tool case for supporting saidtransmitter search coil in said borehole in a position such that theearth formation adjacent said borehole is subjected at least in part tosaid magnetic field; d. a solenoidally wound receiver search coilincluding a glass core which has a coefficient of thermal expansion nogreater than 1.6 X 10 5 inches per inch per *F. said receiver searchcoil being mounted in said case (c) in a position axially spaced apartfrom said transmitter coil (a) a predetermined distance to obtainmagnetic coupling with said transmitter coil at least in part throughthe earth formation adjacent said borehole whereby said alternatingmagnetic field induces in said receiver coil a first alternating voltagesignal dependent in part on the magnetic susceptibility of said earthformation and which is in quadrature phase relationship with thealternating current in said transmitter coil; e. a nulling coil having aprimary winding connected in series with said transmitter coil and asecondary winding connected in series opposition with said receiver coilsaid nulling coil being mounted in said case (c) in a position such thatsaid alternating magnetic field has substantially no affect on saidprimary and secondary winding thereof whereby the induced voltage insaid secondary winding tends to be suBtracted from said receiver coilvoltage rendering detectable the portion of said receiver coil voltagerepresentative of the magnetic susceptibility of the earth formation;and, f. phase sensitive detecting means coupled to said receiver searchcoil for deriving a magnetic susceptibility signal voltage correspondingto the portion of said receiver search coil voltage representative ofthe magnetic susceptibility of the earth formation in quadrature phaserelationship with the alternating current in said transmitter coil. 2.The apparatus of claim 1 wherein the core of said transmitter searchcoil (a) and said receiver search coil (d) comprises a mandrel corecommon to both of said coils wherein each of said coils is a pluralityof electrical windings on said mandrel and wherein the axial distance ofthe nearest winding of said receiver search coil to the nearest windingof said transmitter search coil is in the range of about 200 to 400percent of the axial length of said transmitter search coil on saidmandrel.
 3. The apparatus of claim 1 wherein each of said transmitterand receiver search coils includes a pair of end caps of boron nitridematerial each having an opening adapted to fit closely over said mandrelfor providing axial support for the windings of said transmitter andreceiver coils, said end caps being cemented to said mandrel.
 4. Theapparatus of claim 1 wherein said transmitter coil (a) includes asufficient number of windings to provide said magnetic field as a totalmagnetic field the strength of which is about 20 to 60 Oersteds. 5.Apparatus for logging the magnetic susceptibility of the earth formationadjacent a borehole comprising: a. a glass mandrel having a coefficientof thermal expansion no greater than 1.6 X 10 5 inches per inch per *F;b. a transmitter search coil including a multiplicity of electricalwindings solenoidally wound over said mandrel; c. means including alogging tool case for supporting said mandrel in said borehole in aposition such that the magnetic axis of said transmitter search coil isapproximately parallel with the axis of said borehole; d. an electricalpower supply for providing alternating current power for saidtransmitter search coil whereby an alternating magnetic field isgenerated which passes at least in part through said earth formation; e.a receiver search coil solenoidally wound over said mandrel axiallyspaced apart from said transmitter search coil a predetermined distanceto obtain magnetic coupling with said transmitter coil at least in partthrough said earth formation by a desired penetration, whereby saidalternating magnetic field induces in said receiver coil a firstalternating voltage signal dependent in part on the magneticsusceptibility of said earth formation and which is in quadrature phaserelationship with the alternating current in said transmitter coil; f.at least one end cap of boron nitride material for each of said searchcoils for providing axial support for the electrical windings thereof,each of said end caps including an opening adapted to fit closely oversaid mandrel; g. a nulling coil having a primary winding connected inseries with said transmitter coil and a secondary winding connected inseries opposition with said receiver coil said nulling coil beingmounted in said case (c) in a position such that said alternatingmagnetic field has substantially no affect on said primary and secondarywindings thereof, whereby the induced voltage in said secondary windingtends to be subtracted from said receiver coil voltage renderingdetectable the portion of said receiver coil voltage representative ofthe magnetic susceptibility of the earth formation; and, h. phasesensitive detecting means coupled to said receiver search coil forderiving a magnetic susceptibility signal voltage corresponding to theportion of said receiver search coil voltage representative of themagnetic susCeptibility of the earth formation in quadrature phaserelationship with the alternating current in said transmitter coil.